Pilot signal luminance correction means for video recording apparatus

ABSTRACT

A system is disclosed for correcting luminance errors that are caused by velocity errors in the playback of a color video signal on video recording apparatus of the type which utilizes a pilot that is included in the color video signal at a frequency that is higher than the color subcarrier frequency and a carrier frequency which is frequency modulated by the pilot and color video signal. During playback, luminance correction signals are added to the demodulated color video signal to remove distortions in the luminance level of the color video signal.

The present invention generally relates to video recording apparatusand, more specifically, to a system for providing luminance correctionin a color video signal that is derived from a recording medium in afrequency modulated recording and reproducing system that utilizes apilot frequency signal.

The continued research and development in the field of video recording,particularly videotape recording and playback apparatus, continues toincrease the quality and reliability of the recording and playbackprocess. The recording and playback process requires preciselycontrolled relative movement of the recording medium and the transducingheads which record on and playback from the recording medium. Since theaccuracy required for recording color video signals is extremely high,various compensating or correcting processes and schemes have beendevised to remove errors (time base and amplitude errors) caused byvariations in the relative movement.

Many current videotape recording and reproducing systems utilize thecolor burst signals which occur near the beginning of each televisionline (in both the 525 line NTSC standard used in the United States andthe 625 line PAL standard used in many foreign countries) as a referencefor making time base error corrections since they occur at a knownfrequency and phase and represent the highest repetitive signal samplerate that is inherently available in the color video signal. Since theoff tape color burst signal occurs every 63.5 microseconds, it providesan adequate reference for comparing with an internally generated signalof the same frequency to repetitively correct the off tape color videosignal i.e. the color video signal is continuously corrected, but withinformation derived every 63.5 microseconds. However, there are highrate errors present (those errors which occur between successive colorbursts i.e., velocity errors) which produce disturbances thatdetrimentally affect the video display. The velocity errors have beenapproximately corrected by assuming a linear variation in the reproducedsignal between color bursts.

To increase the basic sampling rate and thereby the correctioncapability, a signal other than the color burst of the television signalmust be employed.

The use of a continuous pilot signal has long been considered to offermany possible advantages for the reason that it allows continuousmonitoring of the off tape color video signal, and error detectingschemes can provide closed loop correction of time base errors, velocityerrors, and amplitude errors. Therefore, the velocity and amplitudeerrors can be more accurately corrected than by assuming linearvariation in the signal within each television line. While pilot signalshave been previously used, many systems have utilized a pilot frequencythat is well below the color burst frequency. While such a low frequencypilot signal would generally be representative of timing errors thatoccurred during the record and playback process, because it is notparticularly close to the color subcarrier frequency of the color videosignal, it is not subjected to all of the distortions that thechrominance signal experiences. The choice of the frequency of the pilotsignal should be optimized so that it is higher than the maximumfrequency of the video signal so as not to unduly limit the pass bandand not be sufficiently high so that it interferes with other circuitoperation, such as beating with the sampling rate of a digital time basecorrecting circuit, for example. Also, if the pilot signal frequency ischosen to be too high, there is a loss of correlation between thevariations in it and the corresponding ones in the chrominance signalitself. Once the pilot frequency is determined, the carrier frequencymust be determined (disregarding for the moment such considerations asamplitude of the pilot, pre-emphasis and the like) so that there is anacceptable signal-to-noise ratio of the pilot relative to the videofrequencies and cross modulation between the pilot and the videofrequencies is not excessive.

When all of these considerations are taken into account to optimize thesystem for use with a pilot, it has been found that the deviation of thecarrier caused by the luminance portion of the color video signal issignificantly reduced and that geometric effects such as velocity errorsand the like can have a perceptible effect on the luminance level of thecolor video signal during playback.

Accordingly, it is an object of the present invention to provide asystem for providing luminance correction of the demodulated videosignal during playback of a recording medium.

A more specific object of the present invention is to provide aluminance correcting signal that is derived from a pilot signal that wasadded to the color video signal prior to recording.

Other objects and advantages will become apparent upon reading thefollowing detailed description, while referring to the attacheddrawings, in which:

FIG. 1 is a schematic block diagram illustrating a record and playbackvideo recording apparatus and including apparatus embodying the presentinvention;

FIG. 2 is a schematic block diagram of a portion of the apparatus shownin FIG. 1 and also embodying the present invention;

FIGS. 3a-3e illustrate waveforms of electrical signals that areproducing during operation at various locations of the schematic blockdiagram shown in FIG. 2;

FIGS. 4a-4d illustrate specific electrical circuitry that may be used tocarry out the operation of the block diagram illustrated in FIG. 2.

Broadly stated, the present invention is directed to a system forproviding a luminance correction signal that can be added to the colorvideo signal to provide correction of velocity and other errors that mayaffect the brightness of the display during playback in a frequencymodulated recording and playback apparatus that utilizes a pilot signalthat is included in the video signal during the recording process. Morespecifically, when the pilot signal is added at a higher frequency thatthe color subcarrier frequency, the carrier frequency that is used torecord the video information signal on tape is adjusted to minimize theinterference effects that have been previously discussed. When such isdone, the frequency deviation of the luminance portion of the colorvideo signal is reduced to either about 0.7 MHz or about 0.85 MHz asopposed to 2.5 MHz and 1.5 MHz for the 525 and 625 line standards,respectively.

The invention is adapted for use with broadcast quality recording andreproducing apparatus and particularly the quadruplex broadcast formatwhich utilizes a head wheel that carries four recording and playbacktransducing heads mounted at 90° with respect to each other, wherein thehead wheel is rotated to either record or playback tracks that aretransversely oriented relative to the longitudinal direction of thetape. The tape is generally about 2 inches wide and is held across itswidth by means of a vacuum in a guide which forms it into an arc of aradius generally corresponding to that of the transducing heads of theguide wheel. During recording or playback, the guide presents the tapeto the rotating heads, causing the heads to penetrate a small distanceinto the tape, i.e., about 0.003 inches, for example, as the heads scanthe tape from edge to edge as the tape is pulled longitudinally past therotating heads and the guide, all of which considerations are well knownin the art.

Since the four heads are rotating to follow successive tracks duringplayback of the tape, the position of the tape relative to the headwheel is important in that geometric misalignment can produce variationsin the signal that is being read. For example, if the effective radiusof the tape changes with respect to the rotating heads, i.e., the bottomportion of the tape guide is spaced away from the heads by a slightamount, a velocity error can be created for the reason that with aconstant angular velocity of the head wheel, a change in the effectiveradius of the tips of the heads relative to the tape will increase ordecrease the head tip velocity. Stated in other words, the end result isthat the velocity varies in proportion to the effective radius of thehead tip to tape contact from the axis of the head wheel so that thepositioning of the guide is extremely important in minimizing velocityerrors of such apparatus, particularly in light of the fact that colorvideo signals are preferably stable within about plus or minus 3nanoseconds.

Such velocity errors can detrimentally affect the luminance informationof the demodulated signal that is derived from the tape during playbackand the present invention provides a luminance correction signal foraddition to the demodulated video signal to correct for normal velocityerrors that may occur due to geometric misalignment and the like.

Turning now to the drawings and particularly FIG. 1, there is shown aschematic block diagram of a videotape recording and playback apparatus,somewhat simplified for the sake of clarity, but embodying the presentinvention. The video signal that is to be recorded is applied at input10 to a color burst gate 12 as well as a notch filter 14 which passesthe video signal except for a small bandwidth at the frequency of 1.5times the color subcarrier frequency which is either 3.58 MHz for NTSCstandard or 4.43 MHz for PAL standard.

The color burst signal from the video signal is applied from burst gate12 to a phase comparator 16 which provides an error signal to a voltagecontrolled oscillator 18 which has a feedback path which phase locks thesignal so that the output from the voltage controlled oscillator islocked to the color burst signal of the video information signal and itis multiplied by a multiplier 20 to produce a pilot signal having afrequency of 11/2 times the color subcarrier frequency of the videosignal. The pilot signal is applied on line 22 to adder 24 which addsthe pilot signal at an amplitude of about 15% relative to the videosignal and the output from the adder is supplied to the modulator 26 anda record amplifier 28 for recording on tape by apparatus which isschematically illustrated to include a rotating head wheel 30 havingfour transducing heads 32 located thereon.

The subsequent reproducing of the signal from the magnetic videotape isperformed by the transducing heads 32 on the rotating head wheel 30 in asimilar manner and the playback signal is applied to a switchingequalizer 36 which compensates for amplitude and other variations thatmay occur to the signal as a result of sequential switching of the heads32 which may have slightly different response characteristics. Theoutput of the switching equalizer 36 is applied to a demodulator 38 andits output is fed to a low pass filter 40 and thereafter to an adder 42before being applied to a digital time base corrector, which isindicated generally at 44, and includes the components within the dottedlines. The time base corrector periodically samples the timing componentassociated with the signal and uses the frequency and phase of the pilotsignal to control the time base compensation of the video signal. Thecontinuous pilot signal provides a continuous reference signal for usein generating the luminance error compensating signal and the time basecorrector also uses the errors that are present in the demodulated pilotsignal to generate error correcting signals for correcting the videosignal so that a corrected video signal appears at its output 46. Thespecific operation of the time base corrector will not be describedherein for the reason that it does not form a part of the presentinvention. However, the pilot signal is used as a reference wherein itsfrequency and phase are used for the time base corrector as previouslymentioned. Referring again to the output of the demodulator 38, it isapplied to a pilot processor 48 via line 50, a band pass filter 49 inline 50', in addition to being applied to the low pass filter 40.Similarly, switching equalizer 36 is adapted to provide a head switchingpulse via line 52 whenever the signal is switched from one transducinghead 32 to an adjacent one during rotation of the head wheel. In theNTSC system, the wheel rotates at an angular velocity of 240 revolutionsper second which means that the head switch occur four times as often,i.e., at a frequency of 960 Hertz. The pilot processor 48 has an inputfrom the adder 42, via line 54, which applies the video signal to thepilot processor and the color burst from the color video signal isutilized for purposes that will be discussed hereinafter. The output ofthe pilot processor includes a luminance correction signal that isapplied to the adder 42 by line 56 for the purpose of providing theluminance correction of the video signal and an output reflecting theoff tape color subcarrier frequency is applied to the time basecorrector by line 58. Additionally, the pilot processor provides achroma amplitude error correcting signal on line 60 which is used by theswitcher equalizer 36 to provide chroma amplitude correction as will bediscussed.

From the foregoing, it is seen that the pilot signal is applied to thepilot processor 48 from line 50' and a luminance correction signal isapplied to the adder 42 via line 56 and the adder adds the luminancecorrection signal to the video signal to thereby correct the luminancefor velocity errors and the like.

To provide the luminance correction, reference is made to FIG. 2 whichis a schematic block diagram of the pilot processor 48 shown in FIG. 1.As is shown therein, the pilot signal from the demodulator is applied online 50' at the lower left hand portion of the drawing. Similarly, thevideo input from adder appears on line 54 and the head switch pulse online 52.

With respect to the operation of the pilot processing circuitry 48, thepilot signal appearing on line 50' is applied to an amplitude modulationdetector 62 through a potentiometer 64 as well as to a narrow band passfilter 66 in the lower path. The upper path containing the AM detector62 provides the automatic chroma amplitude error signal that is appliedto the equalizer 36 for the purpose of maintaining the relativelyconstant chroma amplitude in the video signal that is applied to thedemodulator 38. Since the amplitude of the pilot signal will representthe amplitude of the chroma, the error signal that appears on line 60will represent the chroma amplitude error. The output of the AM detector62 is applied through resistor 68 to an operational amplifier 70 whichhas a capacitor 72 and a resistor 74 connected in parallel across theamplifier input and output. This arrangement defines a control loop forproducing the chroma error that is used to correct the chroma level andit has a frequency response that is compatible with normal variations inthe chroma level that occur, except at the occurrence of head switchingwhich produces a rather abrupt change in the chroma level.

In other words, the bandwidth is chosen so that the frequency responsewill be such that noise is generally disregarded and the changes inamplitude of the chroma level will be followed, except that thefrequency response is not fast enough to follow the abrupt changes thatoccur in the chroma level during head switching. In this regard, thetypical chroma errors that may be present are shown in the waveform ofFIG. 3d which illustrates a series of generally inclined portions 76that may have a duration of about 1 millisecond from head switch to headswitch, with the head switching occurring at the locations 78. It isseen that the chroma error signals have noise thereon which isrepresented by the random deviations in the inclined portions 76 and thetime constant of the loop is such that the random noise is filtered out.The waveform shown in FIG. 3e is illustrative of the signal that occursafter correction for changes in the chroma level. Thus, the correctedsignal is generally constant except for spikes 80 that appearimmediately following head switching but these appear during theblanking period of the television signal and therefore do not affect thetelevision picture.

As previously mentioned, the time constant for the loop is such that thebandwidth of the control loop is relatively narrow so as to disregardnoise but follow normal variations in the chroma level. However, duringhead switching time, provision is made for increasing the time constantof the control loop by switching a resistor 84 which is of a value thatis preferably much lower, i.e. about 10 times lower, to decrease thetime constant of the loop by about 10 times so as to accommodate theabrupt change in the chroma amplitude that occurs during head switchingtime. Thus, the frequency response of the loop is increased by a factorof 10 so that the change in the chroma level can be followed during thehead switching time.

To provide the proper switching action for the control loop, a normallyopen switch 86 is closed when a triggering signal is applied via line88. When the switch 86 is closed, the resistors 68 and 84 are connectedin parallel which changes the time constant of the loop in the manner aspreviously mentioned. To provide the signal on line 88, a pulsegenerator 90 is provided which is triggered by the head switch pulse online 52 and when it is triggered, it produces a 10 microsecond pulse atits output which closes the switch 86 for that amount of time. At thecompletion of the 10 microsecond pulse, the switch 86 is switched backto its normally open condition as shown and the time constant isreturned to its lower value where the frequency response of the loop iscompatible with the normal changes in the chroma amplitude. Referringagain to FIG. 3, the 10 microsecond pulse is shown in the upper leftcorner of the drawing in a representative proportion and the pulsesoccur at every head switch time 78. The transient signals 80 illustratedin FIG. 3e represent the response of the loop with the higher timeconstant and the dotted line representations 92 would approximate thefrequency response that would occur in the event the switch 86 was notclosed and the loop gain not increased in the manner described. In suchinstance, the chroma level would not be constant and would be varyingduring the time that the picture is being displayed, rather than only inthe blanking period as is desired.

To obtain the luminance correction signal output on line 56, the pilotsignal on line 50' from the demodulator via line 50 and band pass filter49 is passed through the narrow band pass filter 66 which preferably hasa pass band of less than about 300 kHz and may be only about 70 kHz andeffectively excludes all frequency components outside of that bandwidth.The pilot signal is passed to a limiter 96, the output of which isapplied to a FM discriminator 98 which has a voltage output that isproportional to the frequency of the input signal on line 100. Theoutput from the FM discriminator 98 is then applied via line 102 to anamplifier 104 through a filter 105 defined by a resistor 106 andcapacitor 108, the time constant of which is chosen to allow for normalchanges in the signal that are caused by velocity errors and the like tobe adequately followed, but not fast enough to correct for large errorsthat would occur at head switch time. Since the output signal on line102 from the FM discriminator is noisy, the filter effectivelyeliminates the noise but permits correction of normal velocity errorsthat may occur in the system. In this regard, reference is made to FIGS.3a and 3b as being illustrative of waveforms that occur at variouslocations in the circuitry as a result of a typical velocity error thatis present in the operation of the apparatus. The error shown in FIG. 3ais generally in the form of a ramp signal wherein the inclined portion110 represents a typical velocity error that results, for example, froma misalignment of a in a videotape recorder guide and wherein thevertical portion 112 is indicative of switching from one head to anotherwhich occurs at a rate of about 960 Hertz since the head wheel isrotated at a 240 Hertz frequency and there are four heads that arelocated on it. In this regard, it should be understood that the 240Hertz frequency relates to the NTSC standard and the 960 Hertz headswitch frequency would occur when four transducing heads are mounted onthe head wheel.

Since the velocity error will be reflected in a deviation of thefrequency of the pilot signal from its predetermined value, the outputof the FM discriminator will have a waveform as shown in FIG. 3b whichcomprises a ramp portion 114 (having noise thereon) and a transientsignal 116 at head switching time. The filter 105 removes the noise fromthe output signal of the FM discriminator, but in order to remove thenoise, it does not have a frequency response that enables it to followthe rapid transition during head switching time. The output of thefilter would generally coincide with the dotted line 118 shown in FIG.3b which does not represent the velocity error with the desiredaccuracy. Therefore, provision is made for altering the frequencyresponse to more accurately coincide with the actual velocity error atthe input of the amplifier 104 and also remove the undesirable noisethat is present in the output of the FM discriminator.

To process the signal that is applied to the amplifier 104 so that it isrepresentative of the velocity error that is actually occurring, such asis shown in FIG. 3a, means are provided to cause the signal to quicklyreach the level that the FM discriminator reaches shortly after headswitch time and to remove the large switching transients that occur (asshown at 116 in FIG. 3b). The processed signal that is applied to theamplifier 104 is shown in FIG. 3c and is derived by utilizing a samplingtechnique that is precisely timed relative to head switching to quicklycharge the capacitor 108 to the low value that it should have.

To achieve the rapid charging of the capacitor 108, the output of the 10microsecond pulse generator 90 is also applied via line 120 to anormally open switch 122 which causes it to close and interconnect thecapacitor 108 with a capacitor 124 which has been charged to a valuethat is approximately at the low value shown at the time interval 126shortly after each head switch. When head switching occurs, the 10microsecond pulse generator causes the switch 122 to be closed for 10microseconds and the capacitor 124 quickly charges capacitor 108 to thevalue stored in the capacitor 124. Thus, closing of switch 122 causescapacitor 108 to quickly acquire the value of capacitor 124. Thisswitching action accomplishes two purposes, i.e., it clamps out theswitching transients so that it does not cause any interference and italso changes the error signal quickly to the value that will appearimmmediately after the switch returns to its open position. When theswitch is opened, the filter is allowed to operate at its normalfrequency response which is fast enough to follow any normal velocityerrors. It should be appreciated that when a recording apparatus is usedthat has multiple heads, it can be desirable to have a separatecapacitor for each of the heads, and a switch for selectivelycommutating the appropriate capacitor to apply a holding voltage to thecapacitor 108 that is accurate for the characteristics of each head.

The voltage level that is present in capacitor 124 is derived from thediscriminator output by a sample and hold technique in the followingmanner. The end of the 10 microsecond pulse produced by the pulsegenerator 90 triggers a 5 microsecond pulse generator 130 which controlsthe operation of a switch 132 which is connected to the output line 102of the FM discriminator via line 134. The switch 132 therebyinterconnects capacitor 124 and the output of the FM discriminator for a5 microsecond time period immediately following the 10 microsecond timeperiod. The capacitor 124 samples the voltage during the small timeinterval 128 of each head sweep and thereby stores an average value ofthe voltage at its lowest value which is the value that is impressedupon the capacitor 108 immediately prior to the sampling time interval.Since the average value is representative of the velocity error that isoccurring as determined by the voltage output of the FM discriminator98, the above described switching action accomplishes the results shownin FIG. 3c wherein a level portion occurs at the beginning of each rampportion and the large switching transient is effectively clamped out tothereby minimize any interference that may be otherwise generated and italso quickly changes the error signal to the value that it should have.While the signal is generally flat in the approximately 15 microsecondperiod following head switch rather than having a slope that isrepresentative of the true velocity error, this time period occursduring the horizontal blanking and does not affect the picture.

The processed signal is amplified by amplifier 104 and appears on line138 which is AC coupled by capacitor 140 to line 56 which provides theluminance correction signal that is added to the color video signal inadder 42.

In addition to providing the luminance correction signal, the pilotsignal is also utilized to provide the reference for performing timebase correction as previously mentioned. After the pilot passes throughthe narrow band pass filter 56 and is limited by the limiter 96, it isapplied to a static phase shifter 144 which has an associated gaincontrol 146 which can be adjusted to the proper setting to reduce thephase shift to zero. However, the phase shifter 144 works in conjunctionwith a dynamic phase shifter 148 which effectively adjusts the phase asa function of voltage that is applied to it. The output of the FMdiscriminator 98 has a voltage that varies in proportion to thefrequency of the pilot and the voltage is applied to control the dynamicphase shifter 148 through a capacitor 150 and line 152. The capacitor150 AC couples the voltage controlled phase shifter since the DCcomponent of the error signal seen subsequently of the band pass filter66 is not particularly significant. Because of the narrow bandwidth,(preferably less than 300 kHz) of the narrow band pass filter 66, avariation in the frequency of the pilot caused by velocity errors andthe like can result in a phase variation or distortion of the pilotsignal which is caused by the narrow band pass filter 66 itself. As aresult of this effect, the phase of the pilot frequency will notaccurately reflect the actual phase of the pilot signal and it is forthis reason that the phase shifters 144 and 148 are used to correct forthe phase error that is introduced by the narrow band pass filteritself. Since the variation in the frequency of the pilot signalproduces an output voltage from frequency discriminator 98 that isproportional to the change to the frequency, the varying voltagecontrolling the dynamic phase shifter 148 via line 152 defines a controlloop that corrects the phase error produced by the band pass filter.

Stated in other words, the voltage of the output of the FM discriminatorvaries in accordance with the changes in the frequency and since thephase distortion produced by the narrow band pass filter is also knownto be a function of the frequency, the voltage output of the FMdiscriminator controls the dynamic phase shifter 148 in a manner wherebythe phase is changed to cancel the phase shift that occurred in the bandpass filter. Since the correcting voltage is in proportion to thechange, the gain control 146 of the static phase shifter 144 caneffectively reduce the phase error produced by the band pass filter 66to zero. Thus, the pilot signal from the dynamic phase shifter 148reflects the phase and frequency deviation that occur in the pilotsignal which is used to control the time base correcter after goingthrough a divider 154 which divides by 11/2 to produce an output signalon line 58 at the subcarrier frequency.

In accordance with another aspect of the system and keeping in mind thatthe pilot signal phase information is used by the time base correcter,it should be appreciated that the exact phase of the pilot may notcorrespond to the phase of the color video signal itself. As a practicalmatter it is unrealistic to assume that all recordings will be made atexactly the same phase relationship, so it is desirable to phase comparethe pilot phase with the phase of the video color burst so that they arephase locked. Another factor that could contribute to a phase error ofthe pilot signal is the temperature sensitivity of circuit componentssuch as the narrow band pass filter and other circuitry that the pilotsignal passes through.

To phase compare the pilot and the color burst signal, the video signalis applied through line 54 to a color burst gate 158 which applies thecolor burst through a band pass filter 159 via line 160 to a phasecomparator 162 which compares the phase of the burst with the phase ofthe pilot from line 164. The output of the phase comparator is a lowfrequency DC voltage level appearing on line 166 which is also appliedto the voltage controlled phase shifter through a resistor 167. Thus,the phase comparing of the color burst with the pilot provides a lowfrequency or DC error correcting voltage which operates over a longperiod of time to lock the color burst with the pilot phase. Theresistor 167 and the capacitor 150 define a filter through which thesignal from the phase comparator 162 is applied to the phase shifter148. It may be desirable to have a separate capacitor 150 for each head,with a commutating switch for selectively connecting them to theresistor, in the event multiple heads are used, to correct fordifferences that exist in the pilot to burst phase relationship for eachhead. The use of the separate capacitors that are commutated enables anaverage correcting value for each head to be applied rather than anaverage value for all heads as is the case when only one capacitor 150is used.

The color burst is also used to perform another function with respect tothe pilot signal before it is applied to the time base corrector 44 andthe output of the burst gate on line 160 also is applied to a presetinput of the divider 154 via line 168. The reason for using the colorburst to preset the divider is that using a multiplier to obtain thepilot frequency of 11/2 times the subcarrier frequency requires amultiplication by 3 (as well as a divide by 2), and the subsequentdivision by 3 (and multiplication by 2) introduces an ambiguity in thesignal in that it can take any one of three distinct phases. Thus, theburst gate 158 is used to generate one or more color bursts to presetthe divider 154 to resolve the ambiguity of the divided subcarrier.

It could also be appreciated that the same phase distortion effects thatare produced by the narrow band pass filter 66 also are occurring inother portions of the circuitry, such as in the time base corrector andin the somewhat wider band pass filter 159 which has a pass band ofwithin the range of about 100 kHz to about 300 MHz. The erroraccumulates throughout the system, partially in the time base correctorand partially in the demodulator 38. To correct for this accumulatedphase error, another error correcting loop is provided by line 170 whichinterconnects line 138 having the luminance correcting signal thereonwith the phase comparator 162. Since the phase comparator 162 comparesthe color burst signal with the color subcarrier frequency, the signalapplied to the phase comparator from line 170 merely modulates thereference of the phase comparator to subtract out whatever errors thathave accumulated in the system. For example, if the head wheel wasrunning too fast, the burst frequency would be too high, and since thecolor burst has gone through some band limiting filters, it would bephase shifted, so there would be a burst phase error. This is applied ina phase that would shift it back and subtract the error and the errorcorrection is a DC component that is applied to the voltage controlledphase shifter 148 through line 166 and 152 in the same manner asdescribed.

Specific electrical circuitry that may be used to carry out theoperation described with respect to the block diagrams of FIG. 2 areshown in FIGS. 4a-4d, which can be grouped together to form a compositedrawing for the circuitry. The numbers shown within the blocks or inparenthesis adjacent thereto represents standard industry numbers forintegrated circuits or TTL logic components and the like. Whereappropriate, pin numbers of the integrated circuits are also illustratedand the reference numbers from the block diagram of FIG. 2 are appliedwhere appropriate. The operation of the circuitry shown in FIG. 4 issubstantially similar to that described with respect to the operation ofthe apparatus schematically illustrated in FIGS. 1 and 2.

From the foregoing description, it should be understood that a system isdisclosed for providing luminance error correction of video signals thatis particularly adapted for use in a system where a pilot signal is usedand, because of practical interference considerations, the pass band forthe luminance portion of the video signal has been reduced to a levelwhich necessitates such luminance correction. The pilot signal is usedto generate the luminance correction signals to compensate for velocityerrors and the like that may detrimentally affect the luminance orbrightness of the television picture during playback of this playback.The luminance correction signal is made to follow normal velocity errorswithout introducing unwanted noise into the video signal, but by reasonof a unique sample and hold scheme is made to accurately follow theabrupt changes that occur in the luminance level during head switchingtime which occurs at frequencies of about 960 Hertz. The slightdeviation of the signal relative to a typical velocity error signalimmediately after head switching does not detrimentally affect the videosignal for the reason that it occurs during the blanking period andtherefore does not affect the picture that is produced.

It is of course understood that although preferred embodiments of thepresent invention have been illustrated and described, variousmodifications, alternatives and equivalents thereof will become apparentto those skilled in the art and, accordingly, the scope of the presentinvention should be defined only by the appended claims and equivalentsthereof.

Various features of the invention are set forth in the following claims.

What is claimed is:
 1. In video recording and reproducing apparatus ofthe type which has at least one transducing head which cooperates torecord on and reproduce from a recording medium and utilizes frequencymodulation of a carrier signal with a video signal, said video signalhaving a pilot signal of predetermined frequency included therein priorto recording, a system for providing luminance error correcting signalsduring reproducing for combining with the video signal, comprising:meansfor recovering said pilot signal from the video signal followingdemodulation; discriminating means associated with said recovering meansfor generating a voltage output that varies in proportion to thefrequency deviation of said pilot frequency; means for filtering saidoutput voltage to eliminate noise therefrom, said filtering means havinga frequency response that enables said filtering means to accuratelyfollow errors introduced as a result of velocity errors that can beproduced by the recording apparatus; and, means for adding the filteredoutput voltage provided by said filtering means to the demodulated videosignal to correct for luminance errors which result from said velocityerrors.
 2. A system as defined in claim 1 wherein said recovering meanscomprises a narrow band pass filter.
 3. A system as defined in claim 1wherein said filtering means comprises a first path having a filter witha frequency response that permits the filter means to follow the errorsintroduced as a result of normal velocity errors, said frequencyresponse being substantially less than the frequency of errorsintroduced as a result of switching between two transducing heads.
 4. Asystem as defined in claim 3 wherein said filtering means furtherincludes:means for adjusting the filtering means to increase thefrequency response thereof to thereby enable the filtering means toapproximately follow the errors introduced by head switching, saidadjusting means being connected to an output of said first path filter,and, switching means associated with said adjusting means to activatethe same in response to a signal being applied thereto, said signaloccurring approximately at the head switching time.
 5. A system asdefined in claim 4 wherein said adjusting means comprises means forsampling and holding the level of said voltage output of saiddiscriminating means immediately following the head switching time, andmeans for forcing the output of said first path filter to the storedlevel before the output thereof would normally reach said level.
 6. Asystem as defined in claim 5 wherein said first path filter comprises afirst capacitor and a resistor.
 7. A system as defined in claim 6wherein said sampling and holding means includes a second capacitor, andsaid switching means includes a first switch interconnecting said secondcapacitor and said first capacitor to charge the first capacitor to thevoltage level on said second capacitor when closed, said first switchresponsive to the activating signal to close for a first predeterminedtime interval immediately after head switching time.
 8. A system asdefined in claim 7 wherein said switching means includes a second switchlocated between said second capacitor and the output of saiddiscriminating means for applying the output voltage of saiddiscriminating means to said second capacitor when closed during asecond predetermined time interval subsequently of each head switchingto thereby provide an average value of the voltage output during saidtime interval over a series of repetitions.
 9. A system as defined inclaim 7 wherein said forcing means includes means responsive to headswitching for generating a switch activating signal for closing saidfirst switch for said first predetermined time interval, said switchactivating signal having a duration corresponding to said firstpredetermined time interval, said duration being about 10 microseconds.10. A system as defined in claim 9 including means for closing saidsecond switch for said second time interval, said second switch closingmeans providing an output adapted to close the same, said output havinga duration of about 5 microseconds.
 11. A system as defined in claim 10wherein said second switch closing means is operatively connected tosaid first switch closing means and is operable in response to the endof said 10 microsecond output.
 12. A system as defined in claim 1wherein the video signal has a subcarrier frequency and said pilotfrequency is 11/2 times the subcarrier frequency of said video signal.13. A system as defined in claim 12 wherein said video signal has asubcarrier frequency of about 3.58 MHz.
 14. A system as defined in claim12 wherein said video signal has a subcarrier frequency of about 4.43MHz.
 15. A system as defined in claim 1 wherein said recovering meanscomprises a band pass filter having pass band within the range of about50 kHz to about 300 kHz centered around the frequency of said pilotsignal.
 16. In video recording and reproducing apparatus of the typewhich has at least one transducing head which cooperates to record onand reproduce from a recording medium and utilizes frequency modulationof a carrier signal with a video signal, said video signal having apilot signal of predetermined frequency included therein prior torecording, a system for providing luminance error correcting signalsduring reproducing for combining with the video signal, comprising:meansfor recovering said pilot signal from the video signal followingdemodulation; discriminating means associated with said recovering meansfor generating a voltage output that varies in proportion to thefrequency deviation of said pilot frequency caused by velocity errors;means operatively connected to said discriminating means for adding theoutput thereof to the demodulated video signal to correct for luminanceerrors which result from said velocity errors.